1. Field of the Invention
The invention relates to optical sampling of tissue in vivo. More particularly, the invention relates to a fiber optic probe placement guide and optical coupler for repeatably sampling a tissue measurement site in vivo.
2. Description of the Prior Art
Noninvasive prediction of blood analytes, such as blood glucose concentration, may employ NIR spectroscopic methods. A commonly assigned application, S. Malin and T. Ruchti, An Intelligent System For Noninvasive Blood Analyte Prediction, U.S. patent application Ser. No. 09/359,191; Jul. 22, 1999 describes a system for noninvasively predicting blood glucose concentrations in vivo, using NIR spectral analysis. Such NIR spectroscopy-based methods utilize calibrations that are developed using repeated in vivo optical samples of the same tissue volume. These successive measurements must yield a substantially repeatable spectrum in order to produce a usable calibration. The heterogeneous and dynamic nature of living human skin leads to sampling uncertainty in the in vivo measurement. Sampling differences can arise due to variable chemical composition and light scattering properties in tissue. As an example: because glucose is not uniformly distributed in tissue, a variation in the volume of tissue sampled is likely to lead to a variation in the strength of the glucose signal, even though glucose concentration in the tissue or blood remains constant. Variation in the placement and replacement of the fiber optic probe used for optical sampling at the measuring surface can lead to sampling in errors in two separate ways: variations in location of the probe can cause a different tissue volume to be sampled; varying the amount of pressure applied to the probe can alter the amount of tissue displaced, causing a larger or smaller tissue volume to be sampled. A change in optical sampling may lead to a variation in the spectral signal for a target analyte even though the concentration of the analyte in the blood or tissue remains unchanged. Furthermore, air gaps between the surface of the fiber optic probe and the surface of the tissue being sampled are another source of sampling error.
Various systems for guiding and coupling fiber optic probes are known. For example, M. Rondeau, High Precision Fiberoptic Alignment Spring Receptacle and Fiberoptic Probe, U.S. Pat. No. 5,548,674; Aug. 20, 1996 and R. Rickenbach and R. Boyer, Fiber Optic Probe, U.S. Pat. No. 5,661,843; Aug. 26, 1997 both disclose fiber optic probe guides utilizing ferrules through which a fiber optic cable or thread is longitudinally threaded. Both devices are connectors that couple fiber optic cables or threads to receptacles in various forms of medical equipment, or to other fiber optic cables. Neither device provides a means for repeatably coupling a fiber optic probe to a tissue measurement site.
T. Kordis, J. Jackson, and J. Lasersohn, Systems Using Guide Sheaths for Introducing, Deploying and Stabilizing Cardiac Mapping and Ablation Probes, U.S. Pat. No. 5,636,634; Jun. 10, 1997 describe a system that employs catheters and guide sheaths to guide cardiac mapping and ablation probes into the chambers of the heart during surgery or diagnostic procedures. The Kordis teachings are directed to surgical methods for the heart, and have nothing to do with optical sampling of tissue in vivo. Furthermore, the apparatus of Kordis, et al. would not be suitable for repeatably coupling a fiber optic probe to a tissue measurement site.
M. Kanne, Laser Mount Positioning Device and Method of Using the Same, U.S. Pat. No. 5,956,150; Sep. 21, 1999 describes a method for using an illumination device, such as a laser to align two components during an assembly process. The Kanne teachings are directed to a manufacturing process rather than optical sampling of tissue in vivo. The Kanne device does not provide any means for repeatably placing a probe guide at a tissue measurement site. It also has no way of monitoring the surface temperature at a tissue measurement site, or of minimizing surface temperature fluctuations and accumulation of moisture at a tissue measurement site.
D. Kittell, G. Hayes, and P. DeGroot, Apparatus for Coupling an Optical Fiber to a Structure at a Desired Angle, U.S. Pat. No. 5,448,662, Sep. 5, 1995 disclose an optical fiber support that is coupled to a frame for positioning an optical fiber at a desired angular position. As with the prior art previously described, the teachings of Kittell, et al. have nothing to do with optical sampling of tissue in vivo. Furthermore, the disclosed device allows an operator to immobilize an optical fiber so that it is maintained in a fixed position, but it does not offer a means of repeatably coupling a fiber optic probe to a tissue measurement site. It also has no way of monitoring the surface temperature at a tissue measurement site, or of minimizing accumulated moisture and temperature fluctuations at the site.
R. Messerschmidt, Method for Non-Invasive Blood Analyte Measurement with Improved Optical Interface, U.S. Pat. No. 5,655,530, Aug. 12, 1997 discloses an index-matching medium to improve the interface between a sensor probe and a skin surface during spectrographic analysis. Messerschmidt teaches a medium containing perfluorocarbons and chlorofluorocarbons. Since they are known carcinogens, chlorofurocarbons (CFC""s) are unsuitable for use in preparations to be used on living tissue. Furthermore, use of CFC""s poses a well-known environmental risk. Additionally, Messerschmidt""s interface medium is formulated with substances that would be likely to leave artifacts in spectroscopic measurements.
It would be desirable to provide a placement guide for a fiber optic probe that coupled the probe to a tissue measurement site for in vivo optical sampling of the tissue. It would also be desirable to provide a means of assuring that the same tissue sample volume may be repeatably sampled, thus eliminating sampling errors due to probe placement. It would also be desirable to provide a way to minimize temperature fluctuations and disperse accumulated moisture at the tissue measurement site, thus eliminating further sources of sampling error. Additionally, it would be advantageous to provide a means of monitoring surface temperature at the tissue measurement site, therefore assuring that the temperature remains constant across repeated optical samples. Finally, it would be highly advantageous to provide an optical coupling fluid to provide a constant interface between a fiber optic probe and the skin at a tissue measurement site that is non-toxic and non-irritating and that doesn""t introduce error into spectroscopic measurements.
The invention provides a fiber optic probe placement guide designed to provide repeatable sub-millimeter location accuracy on the skin surface of a tissue measurement site and a repeatable degree of tissue displacement. The major structural component of the fiber optic probe placement guide is a mount having a probe aperture, into which the fiber optic probe is inserted during use. The contact surface of the mount is curved to approximate the contour of the tissue measurement site, typically a site on a limb of a living subject. The mount incorporates structural features to minimize direct contact between the skin around the tissue measurement site and the contact surface in order to reduce temperature fluctuation and moisture accumulation at the site and on the probe, and to reproduce a small amount of tissue displacement in the vicinity of the tissue measurement site. The fiber optic probe placement guide has crosshair slots that are aligned with crosshairs at the tissue measurement site during repeated placements of the fiber optic probe placement guide in order to minimize optical sampling errors due to placement error. Guideposts on the exterior surface of the fiber optic probe placement guide fit into corresponding guidepost recesses on a subject interface bearing the fiber optic probe to facilitate alignment of the probe with the probe aperture.
During use, the fiber optic probe placement guide is fastened to the tissue measurement site using adhesive or straps. A subject interface bearing a fiber optic probe is directed toward the site; the guideposts are received by the guidepost recesses in the housing of the interface, and the probe is received by the probe aperture. An optical coupling fluid placed on the skin surface at the tissue measurement site eliminates sampling errors due to air gaps between the skin surface and the fiber optic probe.
The fiber optic probe placement guide is also equipped with a temperature probe so that skin temperature in the area directly adjacent the tissue measurement site may be monitored.